1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly to an apparatus and a method for generating a preamble signal for cell identification in an OFDM (Orthogonal Frequency Division Multiplexing) mobile communication system.
2. Description of the Related Art
The current 3G mobile communication system is evolving into a 4G mobile communication system. Unlike previous mobile communication systems which provide simple wireless communication services exclusively, the 4G mobile communication system is being standardized for the purpose of efficient interworking between a wired communication network and a wireless communication network in order to provide integrated wired/wireless communication services at a high speed.
When a signal is transmitted through a wireless channel in the above mobile communication systems, the transmitted signal is subject to multipath interference due to various obstacles existing between a transmitter and a receiver. Characteristics of a wireless channel having multipaths depend on a maximum delay spread and a transmission period of a signal. If the transmission period of the signal is longer than the maximum delay spread, interference may not occur between continuous signals and a frequency characteristic of a channel is determined as frequency nonselective fading.
However, if a single carrier scheme is used when transmitting high-speed data having a short symbol interval, intersymbol interference may increase, causing signal distortion. Thus, the complexity of an equalizer at a user equipment may have to increase in order to effectively deal with this distortion.
To solve the above problem of the single carrier scheme, an OFDM scheme has been suggested.
According to the OFDM scheme, a multi-carrier is used to transmit data. The OFDM scheme is a kind of an MCM (Multi Carrier Modulation) scheme, in which serial symbol arrays are converted into parallel symbol arrays, which are modulated into a plurality of sub-carriers, that is, a plurality of sub-carrier channels which are orthogonal to each other.
The OFDM scheme has been widely used for digital data communication technologies such as digital audio broadcasting (DAB), digital TV broadcasting, wireless local area network (WLAN), and wireless asynchronous transfer mode (WATM). That is, although the OFDM scheme was rarely used before due to its hardware complexity, recent advances in digital signal processing technology including fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT) enable the OFDM scheme to be implemented in the mobile communication system.
The OFDM scheme is similar to a conventional FDM (Frequency Division Multiplexing) scheme, but the OFDM scheme has its unique characteristics. Among other things, the OFDM scheme can transmit a plurality of sub-carriers while maintaining orthogonality among them, thereby obtaining the optimum transmission efficiency when transmitting high-speed data. In addition, since the OFDM scheme makes use of efficient frequency utilization and represents a superior characteristic against multipath fading, it can obtain optimum transmission efficiency when transmitting high-speed data.
More specifically, overlapping frequency spectrums of the OFDM scheme may lead efficient frequency utilization and superior characteristics against frequency selective fading and multipath fading. In addition, the OFDM scheme can reduce an affect of ISI (Intersymbol interference) by using a guard interval, simplify a structure of an equalizer, and reduce impulse-type noise. Thus, the OFDM scheme is widely utilized in various communication systems.
FIG. 1 is a block diagram illustrating a transmitter of a conventional OFDM mobile communication system. The OFDM mobile communication system includes a transmitter 100 and a receiver 150.
The transmitter 100 includes a encoder 104, a symbol mapper 106, a serial to parallel converter 108, an inverse fast Fourier transformer (IFFT unit) 110, a parallel to serial converter 112, a guard interval inserter 114, a digital to analog converter (D/A converter) 116, and an RF (radio frequency) processor 118.
In the transmitter 100, user data 102 including user data bits and control data bits are transmitted to the encoder 104. Upon receiving the user data 102, the encoder 104 codes user data 102 through a predetermined coding scheme and sends the data to the symbol mapper 106. Herein, the encoder 104 may code the user data 102 through a turbo coding scheme or a convolution coding scheme having a predetermined code rate. The symbol mapper 106 modulates coded bits through a predetermined modulation scheme, thereby generates modulated symbols and sends the modulated symbols to the serial to parallel converter 108. Herein, the predetermined modulation scheme includes a BPSK (binary phase shift keying) scheme, a QPSK (quadrature phase shift keying) scheme, a 16 QAM (quadrature amplitude modulation) scheme, or a 64 QAM (quadrature amplitude modulation) scheme.
Upon receiving the serial modulated symbols from the symbol mapper 106, the serial to parallel converter 108 converts the serial modulated symbols into parallel modulated symbols and sends the parallel modulated symbols to the IFFT unit 110. Upon receiving signals from the serial to parallel converter 108, the IFFT unit 110 performs N-point IFFT with respect to the signals and sends the signals to the parallel to serial converter 112.
Upon receiving the signals from the IFFT unit 110, the parallel to serial converter 112 converts the signals into serial signals and sends the serial signals to the guard interval inserter 114. The guard interval inserter 114, which has received the serial signals from the parallel to serial converter 112, inserts guard interval signals into the serial signals and sends the signals to the D/A converter 116. Insertion of the guard interval is necessary to remove interference between a previous OFDM symbol and a current OFDM symbol when OFDM signals are transmitted from an OFDM communication system.
Such a guard interval has been suggested in such a manner that null data with a predetermined interval are inserted into the guard interval. However, when the null data are transmitted into the guard interval, if the receiver erroneously estimates a start point of the OFDM symbol, interference between sub-carriers may occur so that probability of misjudgment for the received OFDM symbol may increase. Thus, a “cyclic prefix” scheme, in which predetermined after bits of an OFDM symbol in a time domain are copied and inserted into an effective OFDM symbol, or a “cyclic postfix” scheme, in which predetermined fore bits of an OFDM symbol in a time domain are copied and inserted into an effective OFDM symbol, is used.
Upon receiving signals from the guard interval inserter 114, the D/A converter 116 converts the signal into an analog signal and sends the analog signal to the RF processor 118. The RF processor 118 includes a filter and a front end unit. The RF processor 118 transmits the signal outputted from the D/A converter 116 to air through a transmit antenna after RF-processing the signal.
Hereinafter, a structure of the receiver 150 will be described. The structure of the receiver 150 is reverse to the structure of the transmitter 100.
The receiver 150 includes an RF processor 152, an analog to digital converter (A/D converter) 154, a guard interval remover 156, a serial to parallel converter 158, a fast Fourier transformer (FFT unit) 160, a channel estimator 162, an equalizer 164, a parallel to serial converter 166, a symbol demapper 168, and a decoder 170.
The signal transmitted from the transmitter 100 is received in the receiver 150 through a receive antenna while noise is being added to the signal when the signal passes through a multipath channel. The signal received through the receive antenna is inputted into the RF processor 152. The RF processor 152 down-converts the signal received through the receive antenna such that the signal has an intermediate frequency band and sends the signal to the A/D converter 154. The A/D converter 154 converts the analog signal of the RF processor 152 into a digital signal and sends the digital signal to the guard interval remover 156.
Upon receiving the digital signal from the A/D converter 154, the guard interval remover 156 removes the guard interval signals and sends serial signals to the serial to parallel converter 158. The serial to parallel converter 158, which has received the serial signals from the guard interval remover 156, converts the serial signals into parallel signals and sends the parallel signals to the FFT unit 160. The FFT unit 160 performs an N-point FFT with respect to the parallel signals outputted from the serial to parallel converter 158 and sends the signals to the equalizer 164 and the channel estimator 162. Upon receiving the signals from the FFT unit 160, the equalizer 164 performs channel equalization with respect to the signals and sends the signals to the parallel to serial converter 166. The parallel to serial converter 166 converts the parallel signals into serial signals and sends the serial signals to the symbol demapper 168.
In the meantime, the signal outputted from the FFT unit 160 is inputted into the channel estimator 162 so that the channel estimator 162 detects pilot symbols or preamble symbols from the signals of the FFT unit 160 and performs channel estimation by using the pilot symbols or the preamble signals. A result of the channel estimation is sent to the equalizer 164. In addition, the receiver 150 generates CQI (channel quality information) corresponding to the channel estimation result and sends the CQI to the transmitter 100 through a CQI transmitter (not shown).
The symbol demapper 168 demodulates the signals outputted from the parallel to serial converter 166 through a predetermined demodulation scheme and sends the decoded signals to the decoder 170. Upon receiving the demodulated signal from the symbol demapper 168, the decoder 170 decodes the demodulated signals through a predetermined decoding scheme, and then, outputs the demodulated signals as final receiving data 172. The demodulation and decoding schemes employed in the receiver 150 are corresponding to the modulation and encoding schemes employed in the transmitter 100.
In the meantime, in a cellular forward communication system employing the above OFDM/OFDMA (Othogonal Frequency Division Multiple Access) schemes, preamble signals or pilot signals, which are preset between the receiver and the transmitter, are used for the channel estimation. That is, the transmitter transmits a signal, which is already-known to the receiver, and the receiver performs the channel estimation based on the already-known signal. The preamble signal including all sub-carriers existing in one symbol interval is used for the channel estimation. Otherwise, the pilot signal used for transmitting relatively high power through at least one sub-carrier forming a predetermined symbol can be utilized for the channel estimation.
The preamble signal or the pilot signal can be used not only for the channel estimation, but also for searching a base station capable of providing optimum signal receiving performance during an initial wireless access and a handoff or for reducing a frame synchronization error in a TDD (Time Division Duplexing) system. The preamble signal signifies a signal transmitted prior to data. The preamble signal can be replaced with a mid-amble signal which is inserted between data symbols to be transmitted. Thus, it is noted that the structure and function of the preamble signal described below can be replaced with those of the preamble signal.
In general, a method of searching a cell site by using the preamble signal includes the following two steps. First, each user equipment receives a preamble signal or a mid-amble signal transmitted from a base station during a downlink transmission interval and performs an FFT (fast Fourier transform) with respect to the preamble signal. Second, a cell list for an initial wireless access or a handoff per each user equipment and a signal to interference and noise ratio (SINR) of a corresponding base station are obtained based on the FFT of the preamble or mid-amble signal.
That is, the preamble signal is used for following objects:
1. Channel estimation,
2. Relative position information estimation for user equipment in a multi-cell, and
3. Measurement for received signal power and SINR.
The preamble signal suggested for the above objects can support a maximum of six cell identifications. In order to generate the preamble signal, each base station selects a predetermined PN (pseudo noise) code and sends it to an IFFT unit. As mentioned above, since the preamble signal can support six cell identifications, six PN codes may be available. In addition, a length of the PN code corresponds to a number of sub-carriers used for the preamble signal.
FIG. 2 is a block diagram showing a structure of a conventional preamble signal receiver for searching a cell identification. When a preamble signal is transmitted from a transmitter (base station) to a receiver (user equipment), the preamble signal is parallel-converted through a serial to parallel converter 201. In addition, the preamble signal is subject to an FFT (fast Fourier transform) through an FFT unit 203 so that the preamble signal is outputted as a frequency domain signal. Then, the frequency domain signal is inputted into a PN code correlator 205 in order to detect a PN code transmitted from the transmitter. The PN code correlator 205 performs a correlation analysis with regard to a plurality of PN codes generated from a PN code generator in order to detect PN codes of the preamble signal. The analysis result of the PN code correlator 205 is inputted into a peak detector 207 and the peak detector 207 detects a peak value of the PN codes, thereby detecting the PN code of the receiving signal. As mentioned above, since the cell sites are identified according to the PN code, the cell site transmitting the signal to the receiver can be detected.
If the channel estimation is achieved by using the preamble signal received in the receiver, an output signal (frequency domain signal) of the FFT unit 203 is multiplied by corresponding PN code bit information of each sample and a resulted value thereof is used as a channel estimation value. At this time, the channel estimation value in a frequency domain, in which the sub-carrier is not used, can be calculated by using an adjacent channel estimation value. In addition, the output sample of the FFT unit 203 can be used when a frame error estimation is performed by using the conventional preamble signal. At this time, a frame synchronization error can be estimated by using a result of a conjugate multiplication for adjacent samples in the frequency domain.
However, the conventional structure for the preamble signal has the following disadvantages.
First, the conventional structure for the preamble signal uses an unique PN code for each base station in order to generate the preamble signal. According to the current standard, only six PN codes can be used for the purpose of cell identification. However, future cellular mobile communication systems may include a relatively large number of cells, so it is necessary to increase the number of PN codes for the cell identification. In order to increase the number of cells, all PN codes and a preamble signal of a modulated time domain must be stored in a memory of each user equipment. In addition, a PN correlation analysis must be carried out with regard to all PN codes during a PN code detection procedure, so a calculation time required for the PN correlation analysis may significantly be increased.
If each base station notifies the user equipment of a neighbor cell list through a predetermined broadcasting channel, a calculation time for a cell search procedure can be reduced without changing an amount of PN codes stored in the memory. However, in this case, a waste of resources can occur at a downlink due to broadcasting of neighbor cell information.
As it is generally known in the art, if a channel received in each user equipment represents a frequency selectivity, a correlation characteristic between PN codes may be degraded. Accordingly, in a cellular network having the above channel environment, each user equipment can receive a plurality of preamble signals during the same time interval. At this time, a cell search function becomes degraded due to degradation of a correlation characteristic between preamble signals received in the user equipment, so it is difficult to precisely measure receiving power of the preamble signal and the SINR of a corresponding base station.
In addition, the conventional structure for the preamble signal represents following disadvantages in view of the channel estimation. The conventional structure for the preamble signal can provide a channel estimator employing various algorithms, such as a channel estimation algorithm per each sub-carrier or a channel estimation algorithm using a frequency window obtained by grouping a plurality of sub-carriers. However, in this case, a set of sub-carriers having a similar channel frequency characteristic within a coherence bandwidth must be inputted into the channel estimator.
If the channel estimator takes a mean value after multiplying PN codes per each sub-carrier, it is possible to obtain an immunity effect for an interference signal, that is, channel estimation performance can be improved through averaging instantaneous Additive White Gaussian Noise (AWGN) peaks and using a low cross-correlation characteristic with regard to preamble signals, which are transmitted from other base stations based on other PN codes. However, according to the channel frequency characteristic in an actual cellular communication environment, the coherence bandwidth is limited within a total frequency bandwidth of at least one sub-carrier, so lengths of some PN codes used for the above operation are shortened. For this reason, it is impossible to obtain the low cross-correlation characteristic between the PN codes.